Solvatochromic absorption band

Then, Dimroth and Reichardt proposed a solvent polarity parameter, Ey(30), based on transition energy for the longest-wavelength solvatochromic absorption band of the pyridynium N-phenolate betaine dye, which is dye No. 30 in a table constructed by these authors. The x(30) values have been determined for more than 360 pure organic solvents and many binary solvent mixtures. 

The major limitation of the x(30) values is the fact that they cannot be measured for acidic solvents such as carboxylic acids. Addition of traces of an acid to solutions of (44) or (45) immediately changes the colour to pale yellow due to protonation at the phenolic oxygen atom of the dye. The protonated form no longer exhibits the long-wavelength solvatochromic absorption band. The excellent linear correlation between iix(30) and Kosower s Z values, which are available for acidic solvents, allows the calculation of t(30) values for such solvents [174]. A further limitation has been the fact that it has not been possible to measure the absorption maximum of the standard betaine dye (44) in the gas phase as a reference state. 

It should be mentioned that the pyridinium A-phenolate betaine dye (44) is not only very sensitive to changes in solvent polarity, but in addition its longest-wavelength solvatochromic absorption band also depends on changes in temperature [73, 175, 180, 208] and pressure [74, 182, 208], on the addition of electrolytes (ionophores) [209-213], as well as on the introduction of substituents in the peripheral phenyl groups cf. Fig. 7-2 in Section 7.1 and reference [332] for a review. 

The addition of electrolytes (ionophores) to solutions of (44) causes hypsochromic shifts of its solvatochromic absorption band [197, 209-213]. This phenomenon can be designated as halo-solvatochromism ) For example, the addition of KI, Nal,... 

Substituents can be introduced not only into the betaine solute, but also into the surrounding solvent molecules. Let us consider a betaine dye solution in various alcohols R-OH with different alkyl groups R. An enhanced hypsochromic shift of the solvatochromic absorption band with increasing hydrogen-bond donor ability of the alcohols should be obtained. This is indeed the case(25). As Figure 9 shows, even exists a linear correlation between the modified E.p-values, measured in nine different alcohols, and Taft s polar substituents constants for the alkyl groups R of these alcohols(25). 

Solvent polarity is one of the most common solvent characteristics that have been used for correlations of rate constants with the nature of the reaction medium. Since relative permittivities and dipole moments did not give satisfactory results, there was a strong need for a microscopic parameter and many attempts have been made to develop empirical solvent parameters that are based on a physical parameter which is sensitive to the solvent polarity. Solvatochromic dyes have been the most successful and particularly Reichardt s t(30) solvent polarity parameter. The latter is based on the transition energy of the longest wavelength solvatochromic absorption band of the betaine dye pyridinium-N-phenoxide (in fact, this is dye number 30 in the first publication ). Its value is given by... 

Solvent Influence. Solvent nature has been found to influence absorption spectra, but fluorescence is substantiaHy less sensitive (9,58). Sensitivity to solvent media is one of the main characteristics of unsymmetrical dyes, especiaHy the merocyanines (59). Some dyes manifest positive solvatochromic effects (60) the band maximum is bathochromicaHy shifted as solvent polarity increases. Other dyes, eg, highly unsymmetrical ones, exhibit negative solvatochromicity, and the absorption band is blue-shifted on passing from nonpolar to highly polar solvent (59). In addition, solvents can lead to changes in intensity and shape of spectral bands (58). 

The merocyanine dye mentioned above shows solvatochromism, which means that the absorption band maximum of the quinoid form (D form) is sensitive to solvent polarity [40,41]. In Fig. 3, the absorption maximum of the solvatochromic band for M-Mc (a low molecular weight merocyanine analog) is plotted against the dielectric constant of 1,4-dioxane/water mixtures [42]. With the relationship... 

Solvatochromic pareuaeters, so called because they were Initially derived from solvent effects on UV/visible spectra, have been applied subsequently with success to a wide variety of solvent-dependent phenomena and have demonstrated good predictive ability. The B jo) scale of solvent polarity is based on the position of the intermolecular charge transfer absorption band of Reichardt s betaine dye [506]. Et(io> values are available for over 200 common solvents and have been used by Dorsey and co-%rarkers to study solvent interactions in reversed-phase liquid chromatography (section 4.5.4) [305,306]. For hydrogen-bonding solvents the... 

The colour of indigo depends dramatically upon its physical state and environment for example, the vapour is red but the colour on the fibre is blue. The marked solvatochromism of indigo (Table 6.4) is attributable mainly to hydrogen bonding. A progressive bathochromic shift of the visible absorption band is observed as the solvent polarity... 

The IR spectra of NDCM salts exhibit cyanide stretching absorption bands in the 2250-2210 cm region and three broad absorption bands associated with the coupling of the v(NO) and v(CC) modes in the 1375-1210 cm region (Table 8). The electronic spectra of the NDCM salts exhibit characteristic jt jt and n n electronic transitions at ca 300 and 480 nm, respectively. In aqueous solution the UV-vis spectra of, e.g., the lithium and barium salts exhibit an additional absorption at 401 nm, which can be attributed to the solvatochromic effect of water. 

Figure 3.49 Examples of solvatochromic plots of absorption spectra, (a) The first absorption band of 4-nitroaniline (charge transfer band), (b) The first absorption band of acetone (n-n band). The ordinates are in units of 103 cm, measured at the band maximum the abcissa are in units of Onsager solvent polarity function f(D)...

Figure 3.50 Outline of the solvatochromic shifts of different absorption bands of the same molecule (e.g. 4-aminobenzophenone). In this case the first transition corresponds to a much larger change in dipole moment, v wavenumber in 103 cm-1 f(D) Onsager polarity function A absorbance...

Figure 3.55 Examples of solvatochromic plots in solvent mixtures, (a) The fluorescence of 4-aminophthalimide in ether/dimethylformamide shows a much steeper non-linearity than its absorption band because of the large dipole moment of the emitting state, (b) The absorption spectrum of an aminobenzene in dioxan/water displays a red shift followed by a steep blue shift at high water concentrations...

Figure 3.57 Solvatochromic plot of the first absorption band of a betaine dye. The transition corresponds to a CT from 0 to the aromatic system. Triangles are protic solvents. E is in kcal moE1 and f(D) is the Onsager function...

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